1. Field of the Invention
The present invention relates generally to surgical instruments used while implanting orthopedic joint prostheses, and relates more particularly to instruments that facilitate implantation of orthopedic knee joint prostheses.
2. Background Information
Implantable orthopedic prostheses, in one form, comprise man-made replacements for the ends and articulating surfaces of the bones of the skeleton. by Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. The knee joint, as a major weight bearing joint, is known to degenerate relatively quickly in the event of abnormality. Also, the knee joint plays a critical role in ambulation and quality of life, resulting in great demand for surgical correction of abnormalities.
To facilitate their implantation, orthopedic knee prostheses have an associated set of specialized surgical instruments, including some that are useful only with a particular prosthesis design, and others that are more generally useful with different prostheses. In general, instruments are provided for cutting and shaping the distal end of the femur, the proximal end of the tibia, and, sometimes, the posterior side of the patella, to prepare those bones to receive prosthetic articulating surfaces. Instruments and jigs for guiding the aforementioned cutting and shaping operations are another important part of the instrument set. Other instruments are used for holding and placing the prosthesis components during surgery. Still another group of instruments is used in the course of surgery for measuring anatomical characteristics and evaluating the progress and accuracy of the surgical operations performed, prior to final implantation of the orthopedic prostheses. The use of such surgical instruments can be comprehended more readily with a basic understanding of knee joint anatomy and the principle knee prosthesis components, as discussed below.
The human knee joint involves three bones: the femur, the tibia and the patella, each having smooth articulation surfaces arranged for articulation on an adjacent articulation surface of at least one other bone. The femur includes at its distal extremity an articulation surface having medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anterior-posterior direction, the condyles joining at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove. The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral convex facets, which facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexion of the knee joint, while the vertical ridge rides within the intercondylar groove to prevent lateral displacement of the patella during flexion. The tibia includes at its proximal end an articulation surface having medial and lateral meniscal condyles that articulate against the medial and lateral condyles, respectively, of the femur. The mutually engaging articulation surfaces of the femur and the patella together form, functionally, the patello-femoral joint, and the mutually engaging articulation surfaces of the femur and tibia together form, functionally, the tibio-femoral joint, which two functional joints together form the anatomical knee joint.
The femur and tibia that comprise the human knee joint are held in proper relationship to each other by soft tissues, i.e., non-bony tissues, that span the joint and are connected to the bones on each side of the joint. Primarily, the soft tissues that constrain and stabilize the knee joint are the ligaments, although the muscles and associated tendons that induce motion in the joint also play a role in stabilizing the joint. In order to preserve the proper relationship and spacing between the femur and tibia, it is important that the artificial articulating surfaces be located at approximately the same location as the natural articulating surfaces. Otherwise, the ligaments that stabilize the knee joint could be either too tight or too loose, or unbalanced between the medial and lateral sides of the joint, adversely affecting the kinematics of the knee, and leading to accelerated wear of the prosthesis.
All or part of one or more of the articulation surfaces of the knee joint may fail to perform properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To accommodate defects of varying scope, while permitting healthy portions of the knee joint to be conserved, a range of types of orthopedic knee implants is available. The range extends from total knee prosthesis systems for replacing the entire articulation surface of each of the femur, tibia and patella, to less comprehensive systems for replacing only the tibio-femoral joint, or only one side (medial or lateral) of the tibiofemoral joint, or only the patello-femoral joint. Commonly employed orthopedic knee prostheses include components that fall within one of three principle categories: femoral components, tibial components, and patellar components. A so-called "total" knee prosthesis includes components from each of these categories. The femoral component replaces the distal end and condylar articulating surfaces of the femur and may include a proximal stem that is received within the medullary canal at the distal end of the femur. The tibial component replaces the proximal end and meniscal articulating surfaces of the tibia and may include a distal stem that is received within the medullary canal at the proximal end of the tibia. The patellar component replaces the posterior side and natural articulating surface of the patella. Sometimes, the patellar component is not used, and the natural articulating surface of the patella is allowed to articulate against the femoral component.
The tibial component of a total knee prosthesis is configured to be received upon and fixed to the proximal end of the tibia. The tibia is prepared to receive the tibial component by resecting a portion of the proximal end of the tibia to leave a substantially horizontal planar bony plateau. Sometimes the exposed medullary canal at the proximal end of the tibia is also reamed to receive a stem portion of the tibial component. The tibial component typically includes a plate portion having an inferior planar surface conforming to the resected bony plateau at the proximal end of the femur. The plate portion may or may not include a depending stem or keel for receipt within a prepared tibial medullary canal. Commonly, a meniscal bearing insert is received atop the plate portion of the tibial component to provide an artificial meniscal articulating surface for receiving the condylar surfaces of the femoral component of the total hip prosthesis. The femoral condylar articulating surfaces articulate against the tibial meniscal articulating surface to restore motion to a defective knee joint.
One known type of tibial component involves a tibial plate made of a bio-compatible metal such as titanium or a titanium alloy, and a meniscal bearing insert made of a bio-compatible polymer such as ultra-high molecular weight polyethylene. The tibial plate is shaped generally as a flat plate having a perimeter that generally conforms to the transverse sectional perimeter of the resected proximal tibia. The tibial plate includes a planar distal, or inferior, surface for engaging the resected proximal tibia, and a proximal, or superior, surface for engaging and receiving the meniscal bearing insert. The bearing insert has an inferior surface that engages the superior surface of the plate portion, and may include locking tabs or other means for fixing the bearing insert to the plate portion against relative movement.
The femoral component of a total knee prosthesis is configured to be received upon and fixed to the distal end of the femur. The femur is prepared to receive the femoral component by resecting a portion of the distal end of the femur to remove the natural condylar articulating surfaces and leave a polygonal resected bone surface. The resected bone surface typically includes three to five intersecting planar surfaces that together form a generally convex, faceted distal surface that mates congruently with a similar concave, faceted proximal surface of the femoral component. Sometimes the exposed medullary canal at the distal end of the femur is also reamed to receive a stem portion of the femoral component. The femoral component typically includes a pair of smoothly curved, highly polished, artificial condylar articulating surfaces that replace the natural condyles of the femur. The condylar articulating surfaces are received upon and articulate against the artificial meniscal articulating surface of the meniscal bearing insert described above. Typically, the femoral component is made of a bio-compatible metal such as titanium, titanium alloy, or cobalt chrome alloy.
Various instrument designs have been proposed for tensioning the ligaments of the knee joint during surgery, by applying a spreading force between the tibia and femur, so that the spacing between the femur and tibia can be ascertained for a given amount of tension. Typically, the spacing/tension relationship is measured independently on both the medial and lateral sides of the joint, to detect any inequality between the medial and lateral soft-tissue ligaments. If one side of the knee joint is found to be more tightly constrained than the other, the tighter side will be released surgically to restore balance to the knee. One disadvantage of prior art instruments is that it is cumbersome to apply and maintain a fixed amount of spreading force to the knee joint, while allowing for detection of unbalance. This is because the spreading force is applied independently to the medial and lateral sides of the joint. It would be advantageous to provide an instrument that would allow a selected amount of spreading force to be applied to the knee joint as a whole, while automatically distributing the force evenly between the medial and lateral sides of the joint, and permitting any unbalance of the knee joint to be readily discerned. This and other desirable advantages are provided by the present invention described below.